LORAN-C is a pulsed, low-frequency (100 Khz) hyperbolic radio navigation system. LORAN-C radio navigation system chains employ three or more synchronized ground stations that each transmit periodic radio frequency pulse trains having, at the respective start of transmissions, a fixed time relationship to each other. The first station to transmit is referred to as the master station, while the other stations are referred to as the secondary stations. The pulse trains are radiated to receiving equipment generally located on aircraft or ships whose positions are to be accurately determined. Each pulse of the pulse trains transmitted by each of the master and secondary stations has an extremely accurate envelope shape, and each pulse train is transmitted at a constant, precise repetition rate called the Group Repetition Interval, with each pulse in a group separated in time from a subsequent pulse by a precise, fixed time interval. In addition, the secondary station pulse train transmissions are delayed a sufficient amount of time after the master station pulse train transmissions to assure that their time of arrival at receiving equipment anywhere within the operational area of the particular Loran-C chain will follow receipt of the pulse train from the master station.
Since the series of pulses transmitted by the master and secondary stations is in the form of pulses of electromagnetic energy which are propagated at a constant velocity, the difference in time of arrival of pulses for a master and a secondary station represents the difference in the length of the transmission paths from the transmitting stations to the Loran-C receiving equipment. The locus of all points on a Loran-C chart representing a constant difference in distance from a master and a secondary station, as indicated by a fixed time difference of arrival of their 100 Khz carrier pulse trains, is a hyperbola. The Loran-C navigation system makes it possible for a navigator to utilize this hyperbolic relationship and precisely determine position using a Loran-C chart on which are located families of hyperbolic curves, each family associated with a particular master-secondary pair of transmitting stations. The modern day Loran-C system provides equipment position location accuracy within 200 feet with a repeatability of within 50 feet.
The detailed operation of the Loran-C radio navigation system is described in a pamphlet put out by the Department of Transportation, U.S. Coast Guard, No. CG-462 dated August, 1974, and entitled "Loran-C User Handbook".
The discrete pulses radiated by each master and each secondary Loran-C transmitter are characterized by extremely precise spacing of 1,000 microseconds between adjacent pulses. Any given point on the precisely shaped envelope of each pulse is also separated by exactly 1,000 microseconds from the corresponding point on the envelope of a preceding or subsequent pulse within the eight pulse trains. To insure such precise time accuracy each master and secondary station transmitter is controlled by a cesium frequency standard clock and the clocks of master and secondary stations are synchronized with each other.
To make the precise time difference of signal arrival measurements required for the Loran-C positional accuracy, the zero crossing of a specific (usually the start of the third) carrier frequency cycle of each pulse must be located. These zero crossings are used to make the time difference of signal arrival measurements in a well-known manner. In theory this will work, but in actual operation noise at and about the same frequency as the carrier frequency of the Loran-C pulses makes the task very difficult. To help locate the third carrier cycle zero crossing, each pulse has an exact pulse shape wherein the maximum positive slope of the pulse envelope is at the third cycle zero crossing. By taking the first derivative of the pulse envelope waveform the maximum positive slope point is found.
A problem exists in the prior art in that the signal strength of received signals very often is weak and in combination with received noise within the passband of the Loran-C receiver results in low signal-to-noise ratios. As the result of low signal-to-noise ratios location of the third carrier cycle zero crossing of each pulse becomes very difficult and many times impossible utilizing existing state of the art equipment. This results in faulty time difference of signal arrival measurements and reliability of the Loran-C equipment is decreased. In addition, received noise distorts the envelope of received pulses and can cause erroneous identification of other than the third cycle zero crossing, thereby causing error in time difference of signal arrival measurements.
The aforementioned problems are exacerbated by the prior art technique of first detecting received signals and then processing the resulting pulse waveform to locate the third cycle zero crossing. As is known to those skilled in the art the detection process decreases the signal-to-noise ratio.
Thus, there is a need in the art for circuitry and techniques that improve the tracking point signal-to-noise ratio of received Loran-C signals and which also improve the ability to find a specified carrier frequency zero crossing to increase the reliability of Loran-C measurements.